Not too surprisingly, breastfeeding and weight loss has not been a huge source of medical investigation. I had questions about this while I was nursing, and found some excellent pieces explaining that the scientific evidence that breastfeeding aids with weight loss is dubious. Check out Amy Kiefer’s take on this at her wonderful blog, expectingscience.com, here. As Kiefer demonstrates, it’s quite clear that women cannot depend on nursing to help them lose weight after giving birth. So where did this idea even come from?It appears the notion was promulgated in the 1970s when breastfeeding numbers took off after decades of low rates.[1] (I discuss this trend briefly in my post about feeding babies milk.) Treating breastfeeding as a weight loss aid was likely a nice selling point for health advocates and feminists alike who were invested in uplifting breastfeeding. The rhetoric still serves the same purpose in the unsettled public discourse about breastfeeding today: besides being a boon for your baby, nursing will help you regain your femininity, your figure, your pre-pregnancy wardrobe, yourself. The idea was – erroneously – “generally accepted” by about 1980, and still is today.[2]

Prior to the 1980s, there was scant professional medical writing on how nursing effects postpartum weight (and almost none on humans). The couple of studies that addressed the topic indicated nursing probably had little, if any, effect on weight loss. One 1957 study of how reproduction influences women’s overall body weight was published in The Journal of Endocrinology. It announced that “lactation had little influence on mean weight.” Breastfeeding consistently “resulted in a small loss during the period of lactation, but its effect was almost eliminated at 24 months after delivery.”[3] A 1975 study that looked at 42 women in either a breastfeeding or a control group indicated that both groups lost “considerable” weight in the six months after they gave birth. (Breastfeeding did promote the break-down of fat, however.)[4] For the most part, serious academic scrutiny of this relationship was confined to the dairy industry; in humans, the issue was “poorly described.”[5]For example, some studies looked at how lactation influenced immediate postpartum weight loss – as in the first week or so after childbirth.[6](This may be an interesting medical query, but most women I know aren’t necessarily interested in this – they want to know whether nursing will help them return to their pre-pregnancy selves any more efficiently.)

More researchers started to explore the relationship between breastfeeding and postpartum weight loss beginning in the 1980s, probably due to a combination of factors. Breastfeeding was much more prevalent, as was the notion that it could aid with weight loss; the science of breastfeeding was expanding; and American physicians were starting to explore the seeds of the nation’s growing weight problems more aggressively. A 1983 study of 31 women in Connecticut concluded – go figure – that calories consumed were the most telling indicator of postpartum weight loss. Because they ate fewer calories, the women who breastfed the least actually lost the most weight! The authors argued that postpartum weight loss boiled down to calories more than anything else, and announced that breastfeeding “does not promote weight loss in well-nourished women.” “Apparently,” the authors surmised, “the state of lactation leads women to consume more calories so that the body weight loss is small.” They were so convinced of their position that they even suggested the recommended daily amounts (RDAs) of food for nursing women be lowered.[7] A separate project in 1988 indicated that a woman’s pre-pregnancy weight was an important determinant of postpartum weight loss. It was unable to weigh in on how lactation influenced postpartum weight change since it did not measure lactation duration at all, but nevertheless mentioned that “previous research” suggested breastfeeding for at least 2 months increased weight loss. The previous research cited, however (one of which was the 1957 study described above, another of which was a project that only measured weight loss eight days after birth, and the other two of which hardly even addressed lactation), was inconclusive.[8]

By the 1990s more thorough investigations were underway, although most still pointed out how difficult the topic was to study. There were some mixed results, but in hindsight things appear clearer. Put simply, none of the work convincingly demonstrated that breastfeeding could reliably help women lose more weight after giving birth. A 1991 research project looked at over 400 women when they were 6 weeks and 12 months postpartum. The results “showed that there was no consistent relationship between weight loss following pregnancy and method of infant feeding.” What mattered most? Weight gain during pregnancy. “It is concluded,” the authors assessed, “that the method of infant feeding, bottle or breast, does not influence weight loss following pregnancy.” However, the authors differentiated breastfeeding women throughout the entire study based on just six weeks. Thus, if a woman nursed for six weeks, that “counted” to put her in the breastfeeding group, along with women who nursed for six months, nine months, or the full year.[9] I love this approach because it mimics many women’s maternity leave, but it also has some obvious shortcomings. Another extended study in the 1990s followed women for 2 years after delivery and divided women into a breastfeeding group (who nursed at least 12 months) and a control group (who nursed less than 3 months). The project authors determined that the breastfeeding group experienced greater weight loss, especially from 3 to 6 months postpartum. But what was the overall difference? At one year, the average was 2 kg (4.5 pounds); the women who breastfed for one year had lost about 4.5 pounds more than formula-feeding mothers, on average (some had gained weight).[10] Hmmm. A 1998 review concluded that weight loss rates didn’t differ for breastfeeding and non-breastfeeding women after birth.[11]

The most recent research puts things into better perspective. A study in 2014 looked at breastfeeding women vs. controls at 6, 9, and 12 months postpartum. The breastfeeding women (who breastfed exclusively for at least 3 months) had lost more weight than their counterparts at all checkpoints – but look at the numbers: at 6 months, they had lost 1.3 pounds more; at 9 months, they had lost 3.7 pounds more, and at 12 months they had lost 3.2 pounds more.[12] Three pounds. That’s it. This strikes me as uninspiring. It’s interesting that this study’s authors chose to highlight that breastfeeding could influence postpartum weight loss, while the authors of a review analysis in the UK used studies just like this to emphasize instead that “there is currently insufficient evidence to suggest that BF [breastfeeding] is directly associated with postpartum weight change.”[13] The review explained that the “methodological rigour of many of the studies [looking at breastfeeding and postpartum weight loss] is questionable.” This may sound unremarkable, but it is a major point of criticism in the scientific community. Ultimately, the project concluded that nursing may help some women lose weight, but definitely not all women; its findings “undoubtedly challenge[d] the common belief portrayed across scientific literature that BF [breastfeeding] promotes weight loss.”[14]

To me, the evidence is uncomplicated: as long as researchers have been studying this, all evidence indicates that breastfeeding does not have a substantial influence on weight loss.

So why does the notion persist? In part, inaccurate ideas about breastfeeding and weight loss probably stem from simple logic. Breastfeeding burns calories. It demands more from your body. But it absolutely does not guarantee weight loss. The caloric expenditures that breastfeeding offers come with a major caveat – women tend to (want to) eat more while nursing. I was ravenous when I was nursing – in a way I never experienced during my pregnancy. I was nearly always hungry and rarely felt satiated; food was never far from my mind. (Although there aren’t formal scientific studies on this, anecdotal evidence and testimony from obstetricians and women also indicates that the last ten or so pounds of pregnancy weight can be very difficult to shed for some breastfeeding women who store fat to help them produce milk.[15] I experienced this myself.) Unfortunately, several outlets – including professional medicine – continue to purport that breastfeeding mothers universally lose more weight, and faster, than their non-breastfeeding counterparts.[16] Others, more admirably, confront the nuances. For example, one ABC piece reported on a 2004 study in The American Journal of Clinical Nutrition concluding that non-breastfeeding women lose body fat more quickly than breastfeeding women. To be fair, this is a real challenge for researchers – it’s difficult to circumscribe breastfeeding groups vs. formula-feeding groups, for one thing. And, there are numerous confounders; women who breastfeed “are systematically different from those women who do not choose to breastfeed.” This, of course, makes it difficult, if not impossible, to tell whether any recorded variations are attributed to breastfeeding. Plus, many of these studies are based on self-reporting; other projects that actually measure women’s weight employ their own scales, and their devices can vary within 0.5 pounds in terms of accuracy. When you are only talking about 1-5 pounds, 0.5 is noteworthy. Not to mention that individual weight can easily vary 2-3 pounds in any given day.

Although the research could change at any time, in this case it seems highly unlikely. The historical trajectory of the work on breastfeeding and postpartum weight loss is quite stable. If breastfeeding does exert any influence on postpartum weight loss, it is very small – this has been true as long as researchers have been studying it. As Amy Kiefer explains, the variations are “so tiny as to be trivial.” They are at most a couple of pounds. “Despite burning a considerable number of calories,” Kiefer says, “breastfeeding has a negligible effect on body fat and total body weight for most well-nourished women.”[17] There are lots of great reasons to breastfeed. There are also lots of great reasons not to. Based on the work that’s been done thus far, postpartum weight loss shouldn’t factor into women’s decisions about infant feeding.